The present invention relates to a non-impact type image recording apparatus and, more particularly, to an image recording apparatus capable of forming an image directly on a plain paper sheet or similar recording medium by use of a powdery toner.
Typical of image recording apparatuses of the type producing hard copies by using a powdery toner are a copier, facsimile machine and laser beam printer which are implemented with an electrophotographic procedure. Specifically, this kind of apparatus has an image carrier in the form of a photoconductive element. The photoconductive element is illuminated imagewise to electrostatically form a latent image thereon. The latent image is developed by a powdery toner, and the resulting toner image is transferred to a recording medium such as a plain paper sheet. The toner image on the recording medium is fixed to complete a recording. In the electrophotographic copier, the imagewise illumination is effected by light which is reflected from or transmitted through a document while, in the laser beam printer, it is effected by a laser beam or an LED (Light Emitting Diode) beam having been modulated by a digital signal. The electrophotographic procedure not only realizes high density and high resolution images but also promotes the use of various kinds of recording media as well as high-speed recording. However, such a procedure is not practicable without resorting to a complicated, bulky and expensive construction, i.e., complicated and accurate optics, photoconductive element which needs intricate management over various physical characteristics thereof, device for transporting a recording medium, a device for transferring a toner image to a recording medium, a device for cleaning the photoconductive element, a drive mechanism which has to drive the entire apparatus with accuracy, etc.
A so-called direct recording system proposed in the past is constrastive to the electrophotographic procedure stated above in that it selectively transfers a toner directly to a recording medium to form a toner image thereon, thereby eliminating the need for optics, photoconductive element, image transferring device, etc. For example, Japanese Patent Laid-Open Publication No. 176655/1983 discloses an apparatus in which a number of needle-like electrodes, a control plate having an opening, and a toner carrier are arranged in this order at intervals. In the apparatus disclosed in this Laid-Open Publication, a recording medium is transported with the back thereof being positioned in close proximity to or in contact with the recording electrodes, while an oscillating electric field is developed between the control plate and the toner carrier to activate a toner. A signal voltage is selectively applied to the recording electrodes to cause the activated toner to fly through the opening of the control plate toward the recording medium. Presumably, the words "activated toner" refer to those toner particles which are moving back and forth, or reciprocating, between the toner carrier and the control plate due to the oscillating electric field. To realize such a condition, it is necessary that the oscillating electric field developed between the control plate and the toner carrier be extremely intense. This in turn requires that a signal voltage intense enough to overcome such an oscillating electric field be applied to the recording electrodes. Otherwise, it would be impossible to transfer the toner in the above condition to a recording medium while controlling its movement and, therefore, to achieve an image with a great S/N ratio. However, such an intense signal voltage is apt to cause discharge to occur between nearby recording electrodes. Should the recording electrodes be located at greater intervals to eliminate the discharge, the resolution would be lowered.
Another image recording apparatus adopting the direct recording system is taught in U.S. Pat. No. 4,431,296. This apparatus is constructed such that an alternating electric field is developed between charging members which are located to face each other, and a developer flies in the space between the charging members.